Panel-form loudspeakers

ABSTRACT

A panel form loudspeaker/microphone combination has a distributed resonant bending wave member and two transducers mounred on the member at sites where the number of vibrationally active resonance anti-nodes is relatively high. One transducer is capable of vibrating the member to couple to and excite resonant bending wave modes in the member and cause the member to produce an acoustic output. The other transducer also is capable of coupling to he resonant bending wave modes in the member, but it produces an output signal in response to resonance of the member due to incident acoustic energy. The combination may be part of a loudspeaker system including an amplifier for the source signal coupled to the first transducer, and a signal receiver coupled to the second transducer for treating the output signal.

This application is a continuation-in-part of Application Ser. No.08/707,012, filed Sep. 3, 1996.

TECHNICAL FIELD

The invention relates to loudspeakers and more particularly toloudspeakers comprising panel-form acoustic radiating elements.

BACKGROUND ART

It is known from GB-A-2262861 to suggest a panel-form loudspeakercomprising:

a resonant multi-mode radiator element being a unitary sandwich panelformed of two skins of material with a spacing core of transversecellular construction, wherein the panel is such as to have ratio ofbending stiffness (B), in all orientations, to the cube power of panelmass per unit surface area (μ) of at least 10;

a mounting means which supports the panel or attaches to it a supportingbody, in a free undamped manner;

and an electro-mechanical drive means coupled to the panel which servesto excite a multi-modal resonance in the radiator panel in response toan electrical input within a 5 working frequency band for theloudspeaker.

DISCLOSURE OF INVENTION

Embodiments of the present invention use members of nature, structureand configuration achievable generally and/or specifically byimplementing teachings of our co-pending application Ser. No.08/707,018. Such members thus have capability to sustain and propagateinput vibrational energy by bending waves in operative area(s) extendingtransversely of thickness often but not necessarily to edges of themember(s); are configured with or without anisotropy of bendingstiffness to have resonant mode vibration components distributed oversaid area(s) beneficially for acoustic coupling with ambient air; andhave predetermined preferential locations or sites within said area fortransducer means, particularly operationally active or moving part(s)thereof effective in relation to acoustic vibrational activity in saidarea(s) and signals, usually electrical, corresponding to acousticcontent of such vibrational activity. Uses are envisaged in co-pendingapplication Ser. No. 08/707,012 for such members as or in “passive”acoustic devices without transducer means, such as for reverberation orfor acoustic filtering or for acoustically “voicing” a space or room;and as or in “active” acoustic devices with transducer means, such as ina remarkably wide range of sources of sound or loudspeakers whensupplied with input signals to be converted to said sound, or in such asmicrophones when exposed to sound to be converted into other signals.

This invention is particularly concerned with active acoustic devices inthe form of loudspeakers. Members as above are herein called distributedmode acoustic radiators and are intended to be characterised as in theabove co-pending patent application and/or otherwise as specificallyprovided herein.

The invention is a panel-form loudspeaker having a distributed modeacoustic radiator and a transducer coupled to vibrate the radiator tocause it to resonate, characterised by a second transducer coupled tothe radiator to produce a signal in response to resonance of theradiator due to incident acoustic energy. The distributed mode acousticradiator may be mounted in a surrounding frame by means of an interposedresilient suspension.

The panel-form loudspeaker may be characterised by at least two saidsecond transducers at spaced locations on the radiator.

The panel-form loudspeaker may be characterised by a further transduceron the radiator to produce a signal in response to resonance of theradiator due to incident acoustic energy, and by means for comparing thesignal generated by the said further transducer with that of those astiff lightweight panel having a cellular core sandwiched between skins.The suspension may be attached to the edge of the panel. The first andsecond transducers may be mounted wholly and exclusively on theradiator. At least two said second transducers may be provided atpredetermined preferential locations or sites on the radiator. A furthertransducer may be provided on the radiator at a predetermined locationor site to produce a signal in response to resonance of the radiator dueto incident acoustic energy, and means may be provided for comparing thesignal generated by the said further transducer with that of thosegenerated by the said second transducer(s). The comparison means maycomprise a signal receiver and conditioner and signal output means.

BRIEF DESCRIPTION OF DRAWINGS

The invention is diagrammatically illustrated, by way of example, in theaccompanying drawings, in which:

FIG. 1 is a diagram showing a distributed-mode loudspeaker as describedand claimed in our co-pending application Ser. No. 08/707,012;

FIG. 2a is a partial section on the line A—A of FIG. 1;

FIG. 2b is an enlarged cross-section through a distributed mode radiatorof the kind shown in FIG. 2a and showing two alternative constructions;

FIG. 3 is a diagram of an embodiment of distributed-mode loudspeakermicrophone according to the present invention, and

FIG. 4 is a perspective view of a piezo-electric transducer.

BEST MODES FOR CARRYING OUT THE INVENTION

Referring to FIG. 1 of the drawings, there is shown a panel-formloudspeaker (81) of the kind described and claimed in our co-pendingapplication Ser. No. 08/707,012 comprising a rectangular frame (1)carrying a resilient suspension (3) round its inner periphery whichsupports a distributed mode sound radiating panel (2). A transducer (9)e.g as described in detail with reference to our co-pending applicationSer. Nos. 09/011,773, 09/011,770, and 09/011,831, is mounted wholly andexclusively on or in the panel (2) at a predetermined location definedby dimensions x and y, the position of which location is calculated asdescribed in our co-pending application Ser. No. 08/707,012, to launchbending waves into the panel to cause the panel to resonate to radiatean acoustic output.

The transducer (9) is driven by a signal amplifier (10), e.g. an audioamplifier, connected to the transducer by conductors (28). Amplifierloading and power requirements can be entirely normal, similar toconventional cone type speakers, sensitivity being of the order of 86-88dB/watt under room loaded conditions. Amplifier load impedance islargely resistive at 6 ohms, power handling 20-80 watts. Where the panelcore and/or skins are of metal, they may be made to act as a heat sinkfor the transducer to remove heat from the motor coil of the transducerand thus improve power handling.

FIGS. 2a and 2 b are partial typical cross-sections through theloudspeaker (81) of FIG. 1. FIG. 2a shows that the frame (1), surround(3) and panel (2) are connected together by respective adhesive-bondedjoints (20). Suitable materials for the frame include lightweightframing, e.g. picture framing of extruded metal e.g. aluminium alloy orplastics. Suitable surround materials include resilient materials suchas foam rubber and foam plastics. Suitable adhesives for the joints (20)include epoxy, acrylic and cyano-acrylate etc. adhesives.

FIG. 2b illustrates, to an enlarged scale, that the panel (2) is a rigidlightweight panel having a core (22) e.g. of a rigid plastics foam (97)e.g. cross linked polyvinylchloride or a cellular matrix (98) i.e. ahoneycomb matrix of metal foil, plastics or the like, with the cellsextending transversely to the plane of the panel, and enclosed byopposed skins (21) e.g. of paper, card, plastics or metal foil or sheet.Where the skins are of plastics, they may be reinforced with fibres e.g.of carbon, glass, Kevlar (RTM) or the like in a manner known per se toincrease their modulus.

Envisaged skin layer materials and reinforcements thus include carbon,glass, Kevlar (RTM), Nomex (RTM) i.e. aramid etc. fibres in various laysand weaves, as well as paper, bonded paper laminates, melamine, andvarious synthetic plastics films of high modulus, such as Mylar (RTM),Kaptan (RTM), polycarbonate, phenolic, polyester or related plastics,and fibre reinforced plastics, etc. and metal sheet or foil.Investigation of the Vectra grade of liquid crystal polymerthermoplastics shows that they may be useful for the injection mouldingof ultra thin skins or shells of smaller size, say up to around 30 cmdiameter. This material self forms an orientated crystal structure inthe direction of injection, a preferred orientation for the goodpropagation of treble energy from the driving point to the panelperimeter.

Additional such moulding for this and other thermoplastics allows forthe mould tooling to carry location and registration features such asgrooves or rings for the accurate location of transducer parts e.g. themotor coil, and the magnet suspension. Additionally with some weakercore materials it is calculated that it would be advantageous toincrease the skin thickness locally e.g. in an area or annulus up to150% of the transducer diameter, to reinforce that area and beneficiallycouple vibration energy into the panel. High frequency response will beimproved with the softer foam materials by this means.

Envisaged core layer materials include fabricated honeycombs orcorrugations of aluminium alloy sheet or foil, or Kevlar (RTM), Nomex(RTM), plain or bonded papers, and various synthetic plastics films, aswell as expanded or foamed plastics or pulp materials, even aerogelmetals if of suitably low density. Some suitable core layer materialseffectively exhibit usable self-skinning in their manufacture and/orotherwise have enough inherent stiffness for use without laminationbetween skin layers. A high performance cellular core material is knownunder the trade name ‘Rohacell’ which may be suitable as a radiatorpanel and which is without skins. In practical terms, the aim is for anoverall lightness and stiffness suited to a particular purpose,specifically including optimising contributions from core and skinlayers and transitions between them.

Several of the preferred formulations for the panel employ metal andmetal alloy skins, or alternatively a carbon fibre reinforcement. Bothof these, and also designs with an alloy Aerogel or metal honeycombcore, will have substantial radio frequency screening properties whichshould be important in several EMC applications. Conventional panel orcone type speakers have no inherent EMC screening capability.

In addition the preferred form of piezo and electro dynamic transducershave negligible electromagnetic radiation or stray magnetic fields.Conventional speakers have a large magnetic field, up to 1 metre distantunless specific compensation counter measures are taken.

Where it is important to maintain the screening in an application,electrical connection can be made to the conductive parts of anappropriate DML panel or an electrically conductive foam or similarinterface may be used for the edge mounting.

The suspension (3) may damp the edges of the panel (2) to preventexcessive edge movement of the panel. Additionally or alternatively,further damping may be applied, e.g. as patches, bonded to the panel inselected positions to damp excessive movement to distribute resonanceequally over the panel. The patches may be of bitumen-based material, ascommonly used in conventional loudspeaker enclosures or may be of aresilient or rigid polymeric sheet material. Some materials, notablypaper and card, and some cores may be self-damping. Where desired, thedamping may be increased in the construction of the panels by employingresiliently setting, rather than rigid setting adhesives.

Effective said selective damping includes specific application to thepanel including its sheet material of means permanently associatedtherewith. Edges and corners can be particularly significant fordominant and less dispersed low frequency vibration modes of panelshereof. Edge-wise fixing of damping means can usefully lead to a panelwith its said sheet material fully framed, though their corners canoften be relatively free, say for desired extension to lower frequencyoperation. Attachment can be by adhesive or self-adhesive materials.Other forms of useful damping, particularly in terms of more subtleeffects and/or mid- and higher frequencies can be by way of suitablemass or masses affixed to the sheet material at predetermined effectivemedial localised positions of said area.

An acoustic panel as described above is bi-directional. The sound energyfrom the back is not strongly phase related to that from the front.Consequently there is the benefit of overall summation of acoustic powerin the room, sound energy of uniform frequency distribution, reducedreflective and standing wave effects and with the advantage of superiorreproduction of the natural space and ambience in the reproduced soundrecordings.

While the radiation from the acoustic panel is largely non-directional,the percentage of phase related information increases off axis. Forimproved focus for the phantom stereo image, placement of the speakers,like pictures, at the usual standing person height, confers the benefitof a moderate off-axis placement for the normally seated listeneroptimising the stereo effect. Likewise the triangular left/rightgeometry with respect to the listener provides a further angularcomponent. Good stereo is thus obtainable.

There is a further advantage for a group of listeners compared withconventional speaker reproduction. The intrinsically dispersed nature ofacoustic panel sound radiation gives it a sound volume which does notobey the inverse square law for distance for an equivalent point source.Because the intensity fall-off with distance is much less than predictedby inverse square law then consequently for off-centre and poorly placedlisteners the intensity field for the panel speaker promotes a superiorstereo effect compared to conventional speakers. This is because theoff-centre placed listener does not suffer the doubled problem due toproximity to the nearer speaker; firstly the excessive increase inloudness from the nearer speaker, and then the corresponding decrease inloudness from the further loudspeaker.

There is also the advantage of a flat, lightweight panel-form speaker,visually attractive, of good sound quality and requiring only onetransducer and no crossover or a full range sound from each paneldiaphragm.

FIG. 3 illustrates a distributed mode panel (2) according to the presentinvention e.g. of the kind shown in FIGS. 1 and 2, intended for use bothas a loudspeaker and as a sound receiver or microphone, e.g. for use inan interactive environment. Although not shown in FIG. 3, the panel (2)is mounted in a surrounding frame (1) and is attached to the frame via aresilient suspension (3) in the manner shown in FIGS. 1 and 2. The frameis suspended on a pair of wires (33), e.g. from a ceiling or on a floorstanding frame (not shown).

The panel is driven to resonate and produce an acoustic output by atransducer (9) of the kind described above with reference to ourco-pending application Ser. No. 09/011,773, 09/011,770, and 09/0011,831which in turn is connected to and driven by an amplifier (10).

The panel also carries a pair of vibration transducers (63) which may bepiezo-electric transducers of the kind shown in FIG. 4 which are coupledin parallel to drive a signal receiver and conditioner (65) connected toan output (66). Another vibration transducer (63) on the panel (2), e.g.of the kind shown in FIG. 4, is coupled to drive a filter/correlator(64) the output from which is fed to the signal receiver and conditioner(65), to provide signal correction.

FIG. 4 shows a transducer (9) for a distributed mode panel (2) in theform of a crystalline disc-like piezo bender (27) mounted on a disc(118), e.g. of brass, which is bonded to a face of the panel (2), e.g.by an adhesive bond (20). In operation an acoustic signal applied to thetransducer (9) via leads (28) will cause the piezo disc (27) to bend andthus locally resiliently deform the panel (2) to launch bending wavesinto the panel.

INDUSTRIAL APPLICABILITY

The invention thus provides a simple loudspeaker/microphone e.g. for usein an interactive environment.

What is claimed is:
 1. A panel form combination loudspeaker/microphonecomprising: a member having selected values of certain physicalparameters which enable the member to sustain and propagate inputvibrational energy in a predetermined frequency range by a plurality ofresonant bending wave modes in at least one operative area extendingtransversely of thickness such that the frequencies of the resonantbending wave modes along at least two conceptual axes of the operativearea are interleaved and spread so that there are substantially minimalclusterings and disparities of spacings of said frequencies, the memberwhen resonating having a plurality of sites at which the number ofvibrationally active resonance anti-nodes is relatively high; a firsttransducer mounted on the member at one of said sites on the member, thefirst transducer being capable of vibrating the member in thepredetermined frequency range to couple to and excite the resonantbending wave modes in the member and cause the member to resonate andproduce an acoustic output; and a second transducer mounted on themember at one of said sites on the member for producing an output signalin response to resonance of the member due to incident acoustic energy,the second transducer adapted to couple to the resonant bending wavemodes in the member and produce an output signal.
 2. A panel-formloudspeaker according to claim 1, wherein the member (2) is mounted in asurrounding frame (1) by means of an interposed resilient suspension(3).
 3. A panel-form loudspeaker according to claim 1 or claim 2,wherein the member (2) has a cellular core (22) sandwiched between skins(21).
 4. A panel-form loudspeaker according to claim 2, wherein thesuspension (3) is attached to the edge of the panel (2).
 5. A panel-formloudspeaker according to claim 1 or claim 4, wherein the first andsecond transducers (9,63) are mounted wholly and exclusively on themember.
 6. A panel-form loudspeaker according to claim 1, wherein atleast two said second transducers (63) are mounted at separate ones ofsaid sites on the member.
 7. A panel-form loudspeaker according to claim6, further comprising a further transducer (63) on the member at anotherone of said sites to produce a signal in response to resonance of themember due to incident acoustic energy, and means (64, 65) for comparingthe signal generated by the said further transducer with those generatedby the said second transducer(s).
 8. A panel-form loudspeaker accordingto claim 7, wherein the comparison means comprises a signal receiver andconditioner (65) and signal output means (66).
 9. A loudspeaker systemcomprising: a member having selected values of certain physicalparameters which enable the member to sustain and propagate inputvibrational energy in a predetermined frequency range by a plurality ofresonant bending wave modes in at least one operative area extendingtransversely of thickness such that the frequencies of the resonantbending wave modes along at least two conceptual axes of the operativearea are interleaved and spread so that there are substantially minimalclusterings and disparities of spacings of said frequencies, the memberwhen resonating having a plurality of sites at which the number ofvibrationally active resonance anti-nodes is relatively high; anamplifier for amplifying a source signal; a first transducer coupled tothe amplifier and mounted on the member at one of said sites on themember, the first transducer being capable of vibrating the member inthe predetermined frequency range to couple to and excite the resonantbending wave modes in the member in response to the amplified sourcesignal and cause the member to resonate and produce an acoustic output;a second transducer mounted on the member at one of said sites on themember for producing an output signal in response to resonance of themember due to incident acoustic energy, the second transducer adapted tocouple to the resonant bending wave modes in the member and produce anoutput signal; and a signal receiver coupled to the second transducerfor treating said output signal.
 10. A panel-form loudspeaker accordingto claim 9, wherein the member is mounted in a surrounding frame bymeans of an interposed resilient suspension.
 11. A panel-formloudspeaker according to claim 10, wherein the suspension is attached tothe edge of the panel.
 12. A panel-form loudspeaker according to claim10 or claim 11, wherein the first and second transducers are mountedwholly and exclusively on the member.
 13. A panel-form loudspeakeraccording to claim 9 or claim 10, wherein the member has a cellular coresandwiched between skins.
 14. A panel-form loudspeaker according toclaim 10, wherein at least two said second transducers are mounted atseparate ones of said sites on the member.
 15. A panel-form loudspeakeraccording to claim 14, further comprising a further transducer on themember at another one of said sites to produce a signal in response toresonance of the member due to incident acoustic energy, and means forcomparing the signal generated by the said further transducer with thosegenerated by the said second transducer(s).
 16. A panel-form loudspeakeraccording to claim 15, wherein the comparison means comprises a signalreceiver and conditioner and signal output means.